X-ray lithography (XRL) has proven to possess great utility in microstructural fabrication. In particular, deep X-ray lithography (DXRL) uses highly collimated x-rays, with energies on the order of several keV, to precisely and accurately transfer a mask pattern into a thick photoresist to define high-aspect-ratio microstructures. Furthermore, LIGA technology (the German acronym for Lithographie, Galvanoformung, and Abformung) combines DXRL with electroplating and plastic molding to enable high volume production of microcomponents from a wide variety of materials, including polymers, metals, alloys, and ceramics. A typical LIGA-based microfabrication process comprises exposing a photoresist to a collimated beam of high energy x-rays through a patterning mask, developing the photoresist to provide a mold, electrodepositing a structural material into the voids of the mold, planarizing the exposed surface of the electrodeposit, and removing the mold to yield the microcomponent.
Prismatically shaped microstructures with nearly arbitrary in-plane geometry and structural heights of several hundred microns to millimeters can thereby be fabricated with submicron dimensional control.
The quality of the results obtained from DXRL-assisted microfabrication is largely determined by the fidelity of the X-ray mask pattern. An X-ray mask consists of an X-ray absorbing layer patterned on a support membrane that is substantially transparent to X-rays. The transparent support membrane can typically be a low-atomic-number material, such as beryllium, carbon, boron nitride, boron carbide, silicon, silicon nitride, or silicon carbide, with thickness of less than 100 μm. The patterned mask absorber contains the information to be imaged onto the photoresist. The absorber is typically a dense, high-atomic-number material, such as gold or tungsten, to attenuate the high energy X-rays in the masked regions.
The requirements of the X-ray absorber are determined by the minimum required exposure contrast, defined as the exposure dose at the photoresist bottom surface in the unmasked exposed regions divided by the exposure dose delivered to the photoresist top surface in the unexposed regions under the is absorber. Exposure contrast is a function of the X-ray source, mask support membrane, mask absorber, and the sensitivity and thickness of the exposed photoresist. Because of the nature of the X-ray absorption, DXRL absorbers are typically much thicker than those used in the integrated circuits industry. In particular, the greater the thickness of the exposed photoresist, the thicker must be the absorber in order to maintain good contrast (e.g., greater than 10) in the photoresist. For a low energy (e.g., several keV) synchrotron X-ray source exposing a several hundred microns thick PMMA photoresist, a gold absorber can have a thickness of several microns or more.
The exposure contrast for a given set of X-ray source parameters, mask absorber and support membrane thicknesses, and photoresist thickness can be obtained using standard models of X-ray attenuation in matter. These attenuation models describe one-dimensional, multi-wavelength transmission through an arbitrary set of X-ray beam filters, transmission through the mask absorber and support membrane, and the subsequent photon flux in the masked and unmasked regions through the photoresist layer thickness.
Recently, there has been increased emphasis on three-dimensional (3D) microstructures, such as are required for 3D photonic crystals and some microoptical elements. Such 3D microstructures, having non-vertical features, can be fabricated by conventional DXRL-assisted processes by using off-normal exposures through a patterned mask. Changing the angle of incidence of the X-ray beam on the photoresist thereby enables the fabrication of non-vertical features having inclined sidewalls. See Ehrfeld et al., “Recent developments in deep x-ray lithography,” J. Vac. Sci. Technol. B16(6), 3526 (1998) and U.S. Pat. No. 5,045,439 to Maner et al.
The 3D mask can either be a proximity mask, with the mask in contact or spaced slightly away from the surface of the photoresist layer, or it can be inclined relative to the plane of the photoresist. Inclined masks can be operationally difficult, particularly when multiple off-normal exposures are required. Using an off-normal exposure geometry with vertical mask holes in a proximity mask works adequately to fabricate many acceptable 3D microstructures. However, the use of vertical mask holes in a thick proximity mask with off-normal exposures can cause absorber shadowing, leading to poor pattern transfer to the photoresist and, therefore, 3D microstructures having geometrically distorted features and tapered sidewalls.
The present invention solves these problems by providing a proximity mask with non-vertical mask holes that are inclined in the direction of the off-normal X-ray beam used to expose the photoresist. The non-vertical mask holes eliminate the geometric distortion and poor sidewall definition caused by absorber shadowing. For multiple off-normal exposures, the mask can be repositioned to realign the mask holes for each subsequent exposure geometry. Alternatively, separate lithographic masks, with different mask hole orientations, can be exchanged between exposures. Alternatively, a master mask can have a plurality of mask holes with different orientations. The aligned mask holes of the master mask can be selected for each exposure with an X-ray blocking selection mask. Alternatively, the absorbing layer of an enhanced mask can be made sufficiently thick so that the X-rays do not penetrate through the absorber in the region of the non-aligned mask holes, eliminating the need for the selection mask. A method to fabricate a precision 3D mask to enable accurate pattern transfer to a thick photoresist layer is also provided.